Magnetic recording devices (HDDs) mainly used in computers and capable of information recording and reproduction are used in various fields such as household video decks, audio apparatuses, and automobile navigation systems for reasons such as large capacities, low costs, high data access speeds, and high data retention reliability. As the use range of the HDDs expands, demands for increasing the storage capacity are increasing. Recently, therefore, the development of high-density HDDs is more and more extensively made.
A so-called perpendicular magnetic recording method is recently mainly used as a magnetic recording method for presently commercially available HDDs. In the perpendicular magnetic recording method, magnetic crystal grains forming a magnetic recording layer for recording information have the axis of easy magnetization in a direction perpendicular to a substrate. Accordingly, the influence of a demagnetizing field between recording bits is little even when the density is increased, and the medium is magnetostatically stable even at a high density. The perpendicular magnetic recording medium generally includes a substrate, a soft magnetic underlayer for concentrating a magnetic flux generated from a magnetic head during recording, a nonmagnetic seed layer and/or nonmagnetic underlayer for orienting magnetic crystal grains of a perpendicular magnetic recording layer in the (00.1) plane, and improving the crystalline orientation, the perpendicular magnetic recording layer containing a hard magnetic material, and a protective layer for protecting the surface of the perpendicular magnetic recording layer.
A granular type recording layer having a so-called granular structure in which magnetic crystal grains are surrounded by a grain boundary region made of a nonmagnetic substance has a structure in which the magnetic crystal grains are two-dimensionally physically isolated by the nonmagnetic grain boundary region. This reduces the magnetic exchange interaction acting between the magnetic grains. This makes it possible to reduce the transition noise in the recording/reproduction characteristics, and reduce the limit bit size. On the other hand, the exchange interaction between the grains is reduced in the granular type recording layer. This often increases the dispersion of a magnetic switching field, which is caused by the composition of the grain and the grain size distribution. As a consequence, the transition noise and jitter noise often increase in the recording/reproduction characteristics.
Also, the lower limit of the recording bit size strongly depends on the magnetic crystal grain size of the granular type recording layer. To increase the recording density of the HDD, therefore, it is necessary to decrease the grain size of the granular type recording layer. An example of the method of decreasing the grain size of the granular type recording layer, there is a method of forming an underlayer having a very small crystal grain size, thereby decreasing the grain size of the granular type recording layer stacked on the underlayer. To decrease the grain size of the underlayer, it is possible to improve a seed layer, granularize the underlayer, or the like.